1. Field of the Invention
The present invention relates to a luminescent nanosheet having a luminescence center element solid-solubilized in a nanosheet in which perovskite octahedral crystal units are linked in a planar configuration.
2. Description of the Prior Art
Development of this type of luminescent nanosheet is now in mounting demand because it allows an excitation source to be more likely to reach the luminescence center element than a conventional particle type luminescent substance.
As taught by Patent Publication 1, retaining the luminescence center element (ion) between nanosheets has been known (see FIG. 8A); however, this has failed in making much use of energy from the excitation source.
For this reason, solid-solubilization of luminescence center element in such a nanosheet as shown in FIG. 8B is still desired.
The nanosheet shown in FIG. 8B has the luminescence center taken and confined in a crystal structure; so it is more efficient than an arrangement with a luminescence center or the like sandwiched between nanosheets in terms of transition of excitation energy from the host or nanosheet to the luminescence center. It could also have been confirmed that luminescence features stabilized with respect to temperature and humidity are achievable because of no need of intermediaries such as water for transition of excitation energy from the host or nanosheet to the luminescence center.
As shown in FIG. 2 of Non-Patent Publication 1, it has been well known that perovskite substances each take various rare earth ions in a perovskite structure as the luminescence center, turning into a luminescent substance. However, Non-Patent Publication 1 does not show solid-solubilization of a rare earth luminescence center between octahedral crystal units of a multistacked crystal sheet structure wherein perovskite octahedral crystal units are stacked over at least 3 high in the vertical direction to a sheet plane.
As set forth in Non-Patent Publications 2 and 3, it has been well known that alkaline metal ions in layered perovskite containing niobium or tantalum may easily ion exchange with other alkaline metal ions (Li+, Na+, Rb+, Cs+) or monovalent ions (NH3+, Ag+, H+, n-C8H17NH3, C5H5NH+, Tl+); however, neither of them shows solid-solubilization of the rare earth luminescence center between the perovskite octahedral crystal units of such a multistacked crystal sheet structure as described above.
Non-Patent Publications 4 and 5 have revealed that the rare earth luminescence center may be doped at not only a rare-earth site but also an alkaline metal site in the layered perovskite containing niobium or tantalum; however, neither of them show solid-solubilization of the rare earth luminescence center between the perovskite octahedral crystal units of such a multistacked crystal sheet structure as described above.
Referring to perovskite-type layers A2Ta3O10 containing tantalum, Non-Patent Publications 6 and 7 have reported that although the elemental composition ratio of A to tantalum is basically 2, the amount of the element at the A site may be decreased (down to 16 mol %) or increased (up to 22.5 mol %) by electrochemical reactions, acid treatments or the like while the perovskite structure is kept intact; however, neither of them again show solid-solubilization of the rare earth luminescence center between the perovskite octahedral crystal units of such a multistacked crystal sheet structure as described above.
Non-Patent Publication 8 has unveiled synthesis of a triple perovskite nanosheet having a quadruple crystal sheet structure wherein perovskite octahedral crystal units are stacked over 4 high in the vertical direction to the sheet plane; however, it does not show solid-solubilization of the luminescence center between the octahedral crystal units.
So far, nanosheets have been obtained by exfoliating a substance having the aforesaid layered structure and dispersing it in a disperse medium, as shown typically in Non-Patent Publications 9 and 10.
In other words, satisfactory dispersion of that substance in the disperse medium has been considered as an essential requirement for obtaining a thinner nanosheet; nanosheets have been created by dispersion using the disperse medium in an amount much larger than the amount of the nanosheet to be dispersed. In addition, such a nanosheet solution has seemed to aggregate; it has been important for utilization of that nanosheet to prevent the once thinned film from gaining thickness by reaggregation.
Therefore, when the obtained nanosheet solution that is of extremely low concentration is used as such, for instance when it is coated on a glass substrate or the like, it would be repelled, resulting in unsatisfactory coating.